1. Field of the Invention
This invention relates to a device for producing quantum effects: a layered composite film incorporating quantum dot devices, which include electrodes controlled by an external energy source. The invention has particular, but not exclusive, application in materials science, as a quantum dot shift register.
2. Description of the Related Art
The fabrication of very small structures to exploit the quantum mechanical behavior of charge carriers e.g., electrons or electron “holes” is well established. Quantum confinement of a carrier can be accomplished by a structure whose linear dimension is less than the quantum mechanical wavelength of the carrier. Confinement in a single dimension produces a “quantum well,” and confinement in two dimensions produces a “quantum wire.”
A quantum dot is a structure capable of confining carriers in all three dimensions. Quantum dots can be formed as particles, with a dimension in all three directions of less than the de Broglie wavelength of a charge carrier. Quantum confinement effects may also be observed in particles of dimensions less than the electron-hole Bohr diameter, the carrier inelastic mean free path, and the ionization diameter, i.e., the diameter at which the carrier's quantum confinement energy is equal to its thermal-kinetic energy. It is postulated that the strongest confinement may be observed when all of these criteria are met simultaneously. Such particles may be composed of semiconductor materials (for example, Si, GaAs, AlGaAs, InGaAs, InAlAs, InAs, and other materials), or of metals, and may or may not possess an insulative coating. Such particles are referred to in this document as “quantum dot particles.”
A quantum dot can also be formed inside a semiconductor substrate through electrostatic confinement of the charge carriers. This is accomplished through the use of microelectronic devices of various design, e.g., an enclosed or nearly enclosed gate electrode formed on top of a quantum well. Here, the term “micro” means “very small” and usually expresses a dimension of or less than the order of microns thousandths of a millimeter. The term “quantum dot device” refers to any apparatus capable of generating a quantum dot in this manner. The generic term “quantum dot,” abbreviated “QD” in certain of the drawings herein, refers to any quantum dot particle or quantum dot device.
The electrical, optical, thermal, magnetic, mechanical, and chemical properties of a material depend on the structure and excitation level of the electron clouds surrounding its atoms and molecules. Doping is the process of embedding precise quantities of carefully selected impurities in a material in order to alter the electronic structure of the surrounding atoms for example, by donating or borrowing electrons from them, and therefore altering the material's electrical, optical, thermal, magnetic, mechanical, or chemical properties. Impurity levels as low as one dopant atom per billion atoms of substrate can produce measurable deviations from the expected behavior of a pure crystal, and deliberate doping to levels as low as one dopant atom per million atoms of substrate are commonplace in the semiconductor industry for example, to alter the band gap of a semiconductor.
Kastner, “Artificial Atoms,” Physics Today (January 1993), points out that the quantum dot can be thought of as an “artificial atom,” since the carriers confined in it behave similarly in many ways to electrons confined by an atomic nucleus. The term “artificial atom” is now in common use, and is often used interchangeably with “quantum dot.” However, for the purposes of this document, “artificial atom” refers specifically to the pattern of confined carriers, e.g., a zero-dimensional electron gas, and not to the particle or device in which the carriers are confined. Kastner describes the future potential for “artificial molecules” and “artificial solids” composed of quantum dot particles. Specifics on the design and function of these molecules and solids are not provided.
Quantum dots can have a greatly modified electronic structure from the corresponding bulk material, and therefore different properties. Quantum dots can also serve as dopants inside other materials. Because of their unique properties, quantum dots are used in a variety of electronic, optical, and electro-optical devices. Quantum dots are currently used as near-monochromatic fluorescent light sources, laser light sources, light detectors including infra-red detectors, and highly miniaturized transistors, including single-electron transistors. They can also serve as a useful laboratory for exploring the quantum mechanical behavior of confined carriers. Many researchers are exploring the use of quantum dots in artificial materials, and as dopants to affect the optical and electrical properties of semiconductor materials.
The embedding of metal and semiconductor nanoparticles inside bulk materials (e.g., the lead particles in leaded crystal) has occurred for centuries. However, an understanding of the physics of these materials has only been understood comparatively recently. These nanoparticles are quantum dots with characteristics determined by their size and composition. These nanoparticles serve as dopants for the material in which they are embedded to alter selected optical or electrical properties. The doping characteristics of the quantum dots are fixed at the time of manufacture and cannot be adjusted thereafter.
In general, the prior art almost completely overlooks the broader materials-science implications of quantum dots. The ability to place programmable dopants in a variety of materials implies a useful control over the bulk properties of these materials. This control could take place not only at the time of fabrication of the material, but also in real time, i.e., at the time of use, in response to changing needs and conditions. However, there is virtually no discussion of the use, placement, or control of programmable quantum dots in the interior of bulk materials. Similarly, there is no discussion of the placement of large arrays of electrically controlled quantum dot devices in one or more layers within a bulk material. There are hints of these concepts in a handful of references, discussed below.
Leatherdale et al., “Photoconductivity in CdSe Quantum Dot Solids,” Physics Review B (15 Jul. 2000), describe, in detail, the fabrication of “two- and three-dimensional . . . artificial solids with potentially tunable optical and electrical properties.” These solids are composed of colloidal semiconductor nanocrystals deposited on a semiconductor substrate. The result is an ordered, glassy film composed of quantum dot particles, which can be optically stimulated by external light sources or electrically stimulated by electrodes attached to the substrate to alter optical and electrical properties. These films are extremely fragile and are “three-dimensional” only in the sense that they have been made up to several microns thick. The only parameter that can be adjusted electrically through changes in the source and drain voltage on the substrate is the average number of electrons in the quantum dots. Slight variations in the size and composition of the quantum dot particles mean that the number of electrons will vary slightly between quantum dots. However, on average the quantum dot particles will all behave similarly.
U.S. Pat. No. 5,881,200 to Burt discloses an optical fiber (1) containing a central opening (2) filled with a colloidal solution (3) of quantum dots (4) in a support medium. See prior art FIGS. 1 and 2 herein. The purpose of the quantum dots is to produce light when optically stimulated, for example, to produce optical amplification or laser radiation. The quantum dots take the place of erbium atoms, which can produce optical amplifiers when used as dopants in an optical fiber. The characteristics of the quantum dots can be influenced by the selection of size and composition at the time of manufacture.
U.S. Pat. No. 5,889,288 to Futatsugi discloses a semiconductor quantum dot device that uses electrostatic repulsion to confine electrons. This device consists of electrodes (16a), (16b), and (17) controlled by a field effect transistor all formed on the surface of a quantum well on a semi-insulating substrate (11). See prior art FIGS. 3A and 3B herein. This arrangement permits the exact number of electrons trapped in the quantum dot (QD) to be controlled by varying the voltage on the gate electrode G. This is useful, in that it allows the “artificial atom” contained in the quantum dot to take on characteristics similar to any natural atom on the periodic table, and also transuranic and asymmetric atoms which cannot easily be created by other means. The two-dimensional nature of the electrodes means that the artificial atom can exist only at or near the surface of the wafer.
Kouwenhoven et al., “Quantum Dots,” Physics World, (June 1998), describe the process of manipulating an artificial atom confined in a similar device, including changing its atomic number by varying the voltage on a gate electrode. The described device is capable of holding up to 100 electrons, whose “periodic table” is also described, and is different from the periodic table for normal atoms since the quantum confinement region is nonspherical. The materials science implications of this are not discussed.
Turton, “The Quantum Dot,” Oxford University Press (1995), describes the possibility of placing such quantum dot devices in two-dimensional arrays on a semiconductor microchip as a method for producing new materials, for example, through the combination of adjacent artificial atoms as “molecules.” This practice has since become routine, although the spacing of the quantum dot devices is typically large enough that the artificial atoms formed on the chip do not interact significantly nor do they produce macroscopically significant doping effects.
Goldhaber-Gordon et al., “Overview of Nanoelectronic Devices,” Proceedings of the IBEE, Vol. 85, No. 4, (April 1997), describe what may be the smallest possible single-electron transistor. This consists of a “wire” made of conductive C6 benzene molecules with a “resonant tunneling device,” or “RTD,” inline that consists of a benzene molecule surrounded by CH2 molecules, which serve as insulators. The device is described, perhaps incorrectly, as a quantum well (rather than a quantum dot) and is intended as a switching device transistor rather than a confinement mechanism for charge carriers. However, in principle the device should be capable of containing a small number of excess electrons and thus forming a primitive sort of artificial atom. Thus, the authors remark that the device may be “much more like a quantum dot than a solid state RTD.” See p. 19. The materials science implications of this are not discussed.
U.S. Pat. No. 6,512,242 to Fan et. al. describes a device for producing quantum effects comprising a quantum wire (504), energy carried along the quantum wire under voltage control, and quantum dots (502, 503) near the quantum wire that hold energy. The quantum wire transports electrons into and out of a quantum dot or plurality of quantum dots through “resonant tunneling” rather than through any direct connection between the quantum wire and the quantum dot. As described by Fan et al., the quantum dots serve as “resonant coupling elements” that transport electrons between the quantum wire acting as an electronic waveguide or between different ports on the same waveguide. In other words, the quantum dots serve as a kind of conduit. However, there is no means for controlling the number of electrons trapped inside the quantum dots at any given time, nor for controlling the size or shape of any artificial atom that might briefly and incidentally exist there.
U.S. Patent Application Publication US 2002/0079485 A1 by Stinz. et. al discloses a “quantum dash” device that can be thought of as an asymmetric quantum dot particle with elongated axes, or as a short, disconnected segment of quantum wire. In this sense, quantum dashes are merely a special class of quantum dot particles. As described by Stinz et al., a plurality of the quantum dash devices are embedded at particular locations inside a solid material to enhance the excitation of laser energy within the material. The resulting structure is a “tunable laser” with an output frequency that can be adjusted over a narrow range. This tuning is accomplished through “wavelength selective feedback” using an external optical grating to limit the input light frequencies that can reach the dashes inside the material. The publication notes that “an ensemble of uniformly sized quantum dashes that functioned as ideal quantum dots would have an atomic-like density of states and optical gain.” Stinz et al. relies on the exact geometry and composition of the semiconductor material to produce quantum dashes of a particular size and shape. Therefore, selection of the available quantum states is achieved exclusively at the time of manufacture, “with a variety of length-to-width-to-height ratios, for example, by adjusting the InAs monolayer coverage, growth rate, and temperature.” While a beam of photons with carefully selected energies can excite these charge carriers inside the quantum dashes, it cannot alter the fixed size or shape of the quantum dashes. The energy affects all the quantum dashes equally, along with the surrounding material in which they are embedded. Furthermore, if the surrounding material is opaque, then photon energy cannot reach the quantum dashes at all.
U.S. Patent application No. US 2002/0114367 A1 by Stinz et. al. discloses “an idealized quantum dot layer that includes a multiplicity of quantum dots embedded in a quantum well layer sandwiched between barrier layers.” Similarly, U.S. Pat. No. 6,294,794 B1 to Yoshimura et. al. discloses “a plurality of quantum dots in an active layer such that the quantum dots have a composition or doping modified asymmetric in a direction perpendicular to the active layer.” These quantum dot particles are simply embedded in an optical crystal. Yoshimura et. al. suggest the use of quantum dots as dopants and introduce the concept of asymmetric dopants with nonlinear effects. A similar quantum dot layer structure is disclosed in U.S. Pat. No. 6,281,519 B1 to Sugiyama et. al.
McCarthy, “Once Upon a Matter Crushed,” Science and Fiction Age (July 1999), in a science fiction story, includes a fanciful description of “wellstone,” a form of “programmable matter” made from “a diffuse lattice of crystalline silicon, superfine threads much finer than a human hair,” which use “a careful balancing of electrical charges” to confine electrons in free space, adjacent to the threads. This is probably physically impossible, as it would appear to violate Coulomb's Law, although we do not wish to be bound by this. Similar text by the same author appears in McCarthy, “The Collapsium,” Del Rey Books (August 2000), and McCarthy, “Programmable Matter,” Nature (5 Oct. 2000). Detailed information about the composition, construction, or functioning of these devices is not given.
U.S. patent application Ser. No. 09/964,927 by McCarthy et. al. discloses a fiber incorporating quantum dots as programmable dopants, and discusses the use of such fibers in materials science, either by embedding one or more fibers inside a bulk material or by braiding, stacking, or weaving the fibers together. The application discloses an embodiment wherein the fibers are flat ribbons. Prior art FIGS. 4A and 4B show a fiber containing control wires (34) in an insulating medium (35), surrounded by a quantum well, plus an optional memory layer (33). The quantum well is formed by a central or transport layer (32) of a semiconductor (similar to the negative layer of a P—N—P junction) surrounded by barrier or supply layers (31) of a semiconductor with higher conduction energy (similar to the positive layers of a P—N—P junction). The surface of the fiber includes conductors that serve as the electrodes (30) of a quantum dot device. These electrodes (30) confine charge carriers in the quantum well into a small space or quantum dot (QD) when a reverse-bias voltage is applied, since the negative charge on the electrodes (30) repels electrons, preventing their horizontal escape through the transport layer (32). The electrodes (30) are powered by control wire branches (36) reaching to the surface of the fiber from the control wires (34) in the center of the fiber. Once the charge carriers are trapped in a quantum dot (QD), they form an artificial atom that is capable of serving as a dopant. Increasing the voltage on the electrodes (30) by a specific amount forces a specific number of additional carriers into the quantum dot (QD), altering the atomic number of the artificial atom trapped inside. Conversely, decreasing the voltage by a specific amount allows a specific number of carriers to escape to regions of the transport layer (32) outside the quantum dot (QD). Thus, the doping properties of the artificial atom are adjusted in real time through variations in the signal voltage of the control wires (34) at the fiber's center.
Hennessy et al., “Clocking of molecular quantum-dot cellular automata,” J. Vac. Sci. Technol. B, pp. 1752-1755 (September/October 2001), disclose a one-dimensional shift register composed of quantum dots, for use in computer logic and memory. The items shifted are binary bits of information, represented by single electrons in the dots. Such single electrons could be regarded as a degenerate form of artificial atom (i.e., an artificial “hydrogen” atom), although the structures are not described that way. Any materials science implications of the electron confinement is not addressed. The system described is neither intended for nor capable of shifting artificial atoms of arbitrary size, shape, or energy level.
The information included in this Background section of the specification, including any references cited herein and any description or discussion thereof, is included for technical reference purposes only and is not to be regarded subject matter by which the scope of the invention is to be bound.